Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau
Joel Saylor*Peter DeCellesJay QuadeDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
74
Geosphere; April 2010; v. 6; no. 2; p. 74–92; doi: 10.1130/GES00507.1; 12 fi gures; 1 table; 2 supplemental tables.
*Present address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712-0254, USA.
ABSTRACT
The Zhada basin is a large Neogene extensional sag basin in the Tethyan Hima-laya of southwestern Tibet. In this paper we examine environmental changes in the Zhada basin using sequence stratigraphy, isotope stratigraphy, and lithostratigraphy. Sequence stratigraphy reveals a long-term tectonic signal in the formation and fi lling of the Zhada basin, as well as higher-frequency cycles, which we attribute to Milankovitch forcing. The record of Milankovitch cycles in the Zhada basin implies that global cli-mate drove lake and wetland expansion and contraction in the southern Tibetan Pla-teau from the Late Miocene to the Pleisto-cene. Sequence stratigraphy shows that the Zhada basin evolved from an overfi lled to underfi lled basin, but continued evolution was truncated by an abrupt return to fl uvial conditions. Isotope stratigraphy shows dis-tinct drying cycles, particularly during times when the basin was underfi lled.
A long-term environmental change observed in the Zhada basin involves a decrease in abundance of arboreal pollen in favor of nonarboreal pollen. The simi-larity between the long-term environmen-tal changes in the Zhada basin and those observed elsewhere on and around the Tibetan Plateau suggests that those changes are due to global or regional climate change rather than solely the result of uplift of the Tibetan Plateau.
INTRODUCTION
Uplift of the Tibetan Plateau has long been viewed as a major forcing factor in regional and global climate change (e.g., Raymo and Ruddi-
man, 1992; Molnar et al., 1993; France-Lanord and Derry, 1994; Ruddiman et al., 1997; An et al., 2001; Abe et al., 2005; Molnar, 2005). Uplift is also thought to have directly driven environmental change on the Tibetan Plateau (e.g., Liu, 1981a; Zhang et al., 1981; Zhu et al., 2004; Wang et al., 2006). However, recent work suggests that global climate change drives climate and environmental change on the Tibetan Plateau (e.g., Dupont-Nivet et al., 2007). Moreover, uplift histories of the Tibetan Plateau based on faunal or fl oral associations differ signifi cantly from those based on stable isotope and other quantitative paleoelevation studies. Paleofl oral assemblages from Pleisto-cene deposits on the Tibetan Plateau are simi-lar to modern fl oral assemblages at low eleva-tions (e.g., Axelrod, 1981; Xu, 1981; Zhang et al., 1981; Li and Zhou, 2001a, 2001b; Meng et al., 2004; Molnar, 2005; Wang et al., 2006) and are used to argue for plateau uplift of at least 1 km since the Late Miocene. A similar argu-ment is based on the abundance of mammal megafauna on the Tibetan Plateau in the Late Miocene–Pliocene and their relative paucity now (e.g., Cao et al., 1981; Zhang et al., 1981; Li and Li, 1990; Meng et al., 2004; Y. Wang et al., 2008a). In contrast, other lines of evidence indicate that the southern Tibetan Plateau has been at high elevations since at least the Mid-dle Miocene (Garzione et al., 2000a; Rowley et al., 2001; Spicer et al., 2003; Currie et al., 2005; Saylor et al., 2009) and central Tibetan Plateau since at least the Oligocene (Cyr et al., 2005; Graham et al., 2005; Rowley and Cur-rie, 2006; DeCelles et al., 2007; Dupont-Nivet et al., 2008). These paleoelevation studies also show that uplift predated widespread Late Miocene climate change (see Molnar, 2005, for a summary of evidence for Late Miocene climate change). These studies call into ques-
tion the direct link between uplift and environ-mental change on the Tibetan Plateau.
The environmental effects of tectonics and climate change can best be addressed in basins that contain all of the proxies mentioned above: pollen, leaf fossils, mammal fossils, and carbon-ates used in stable isotope studies. A case in point is the Zhada basin in southwestern Tibet. However, the lack of a coherent, comprehensive basin analysis integrating all the paleoenviron-mental proxies has hampered efforts to untangle the climatic and tectonic signals in the Zhada record. The Zhada Formation is described as both upward fi ning (Zhang et al., 1981; Zhou et al., 2000; Li and Zhou, 2001b) and capped by boulder conglomerates (Zhu et al., 2004; Zhu et al., 2007). There is similarly little con-sensus regarding the basin’s tectonic origin. The Zhada basin is presented as having devel-oped in the hanging wall of the low-angle South Tibetan detachment system or as a half-graben produced in response to arc-normal extension (Wang et al., 2004; S.F. Wang et al., 2008a). It is also proposed to be a fl exural basin respond-ing to arc-perpendicular compression (Zhou et al., 2000). The presence of capping boulder conglomerates has led to the suggestion that the basin was recently uplifted (Zhu et al., 2004). Until recently, the Zhada basin was understood to have been at low elevations until as late as the Pleistocene (e.g., Zhang et al., 1981; Zhou et al., 2000; Li and Zhou, 2001a; Zhu et al., 2004).
In a recent paper (Saylor et al., 2009) we documented the chronostratigraphy and stable isotope record of the Zhada basin. Here we pro-vide basin-wide lithologic and sequence strati-graphic correlations, frequency analysis of the record of environmental change, and a detailed isotope stratigraphy. Our results suggest that global climate change, possibly in conjunc-tion with regional climate change, controlled
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 75
Main C
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South Tibetan Detachment
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Zhada Basin
Great Counter
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Indus-Yalu Suture Zone
Indi
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Ayi Shan
Pulan Basin
Leo Pargil Detachment Great Counter
thrust
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Figure 1. (A) Elevation, shaded relief, and generalized tectonic map of the Himalayan-Tibetan orogenic system showing the location of the Zhada basin relative to major structures. (B) Generalized geologic map of the Zhada region. Modifi ed from mapping by Cheng and Xu (1987), Murphy et al. (2000, 2002), and mapping by M. Murphy (2005, 2006, 2007, personal commun.).
Saylor et al.
76 Geosphere, April 2010
environmental variability in the southwestern Tibetan Plateau during the Late Miocene– Pleistocene. The data also point to the possi-bility of establishing a high-resolution climate record for this high-elevation basin extending from the Pleistocene to the Miocene.
REGIONAL GEOLOGICAL SETTING
The Zhada basin is the largest late Cenozoic sedimentary basin in the Himalaya. It is located just north of the high Himalayan ridge crest in the western part of the orogen (~32°N, 82°E; Fig. 1A). The basin is at least 150 km long and 60 km wide, and the current outcrop extent of the basin fi ll is at least 9000 km2 (Fig. 1B).
The Zhada basin is located in a zone of active arc-parallel extension (Ni and Barazangi, 1985; Zhang et al., 2000; Murphy et al., 2002; Thiede et al., 2006; Valli et al., 2007; Murphy et al., 2009). It is bounded by the South Tibetan detachment system to the southwest, the Indus suture to the northeast, and the Leo Pargil and Gurla Mandhata gneiss domes to the northwest and southeast, respectively (Fig. 1B). The role of each of these structures in the development of the Zhada basin is an area of ongoing research. The South Tibetan detachment system is a series of north-dipping, low-angle, top-to-the-north normal faults that place low-grade metasedi-mentary rocks of the Tethyan sequence on high-grade gneisses and granites of the Greater Himalayan sequence. Along strike, both to the east and west, ages for movement on the South Tibetan detachment system range from 21 to 12 Ma (Hodges et al., 1992, 1996; Noble and Searle, 1995; Searle et al., 1997; Murphy and Harrison, 1999; Searle and Godin, 2003; Cottle et al., 2007). To the northeast of the Zhada basin, the Oligocene–Miocene Great Counter thrust, a south-dipping, top-to-the-north thrust system, cuts the Indus suture (e.g., Gansser, 1964; Yin et al., 1999; Murphy and Yin, 2003). Exhumation of the Leo Pargil and Gurla Mandhata gneiss domes (Fig. 1B) by normal faulting began 9–10 Ma (Zhang et al., 2000; Murphy et al., 2002; Thiede et al., 2006) and continues today.
The Zhada Formation occupies the Zhada basin and consists of >800 m of fl uvial, lacus-trine, eolian, and alluvial fan deposits. The sedi-mentary basin fi ll is undisturbed and forms an angular or buttress unconformity with under-lying Tethyan sequence strata that were previ-ously shortened in the Himalayan fold-thrust belt (Saylor, 2008). The Zhada Formation is capped by a geomorphic surface that extends across the basin and is interpreted as a paleode-positional plain that marks the maximum extent of sediment aggradation prior to integration of the modern Sutlej River drainage network.
800
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CS SS Cgm
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ght (
m)
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Age(Ma)
Globalpolarity
timescale
South Zhada Section
PolarityChrons
1n
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3Bn
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5n
C3/C4 Transition
After deposition, the basin was incised to base-ment by the Sutlej River, exposing the complete thickness of the Zhada Formation. The best estimate for the age of the Zhada Formation is between ca. 9.2 and after 1 Ma, based on ver-tebrate fossils and magnetostratigraphy (Fig. 2) (Lourens et al., 2004; S.F. Wang et al., 2008b; Saylor et al., 2009).
METHODS
Sedimentology
We measured 14 stratigraphic sections span-ning the basin extent from the Zhada county seat in the southeast to the Leo Pargil Range front in the northwest (Fig. 1B). Sections were mea-sured at centimeter scale.
Correlations
The geomorphic surface that caps the Zhada Formation is correlative across the basin, and provides the datum for sequence stratigraphic and lithologic correlation. Correlations are based on major stratigraphic members that can
be physically traced (Saylor, 2008). Magneto-stratigraphy linking the South Zhada, South-east Zhada, and East Zhada sections provides additional constraints. A fi nal independent constraint is the switch from exclusively C3 to mixed C3 and C4 vegetation that is observed between 130 and 230 m in the South Zhada section and at ~300 m in the East Zhada sec-tion (Saylor et al., 2009). The expansion of C4 vegetation is observed across the Indian sub-continent and southern Tibet ca. 7 Ma (Quade et al., 1989, 1995; France-Lanord and Derry, 1994; Garzione et al., 2000a; Ojha et al., 2000; Wang et al., 2006).
Frequency Analysis of Zhada Formation Cycles
The sedimentological record of the Zhada Formation archives the cyclical expansion and contraction of a large paleolake. Frequency analysis was conducted by spectral analysis and also by calculation of the average duration of cycles. In order to apply spectral analysis to this record, a waveform was created by assign-ing numerical values to each of the depositional
Figure 2. South Zhada lithologic section and associated magnetostrati-graphic section and correlation to the geomagnetic polarity time scale (GPTS) of Lourens et al. (2004). VGP—virtual geomagnetic pole; C—claystone; S—siltstone; SS—sandstone; Cgm—conglomerate.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 77
environments as follows: 5—fl uvial and alluvial fan associations; 4—supralittoral associations; 3—littoral associations; 2 or 1—profundal associations, based on the presence or absence of terrestrial clastic or plant material, respec-tively. Depositional environments in the South Zhada measured section were identifi ed at 0.5 m increments or where the depositional environ-ment changed. The series was converted from the depth domain to the time domain by linear interpolation between magnetostratigraphic tie points, justifi ed by the generally linear subsidence/sediment accumulation rates (Say-lor, 2008). The assumption of a linear sediment accumulation rate likely breaks down at short time scales, implying that interpretation of cycles <100 k.y. must await a more fi nely tuned basin chronology. The result is a clipped wave-form with uneven sample spacing and temporal resolution better than 4 k.y. (Fig. 3; Supple-mental Table 11). Progradation of basin margin depositional environments leads to waveform saturation and loss of resolution at ages younger than 3.3 Ma. In order to evaluate the effect of this saturation on spectral analysis, both the 5.23–2.581 Ma and the 5.23–3.3 Ma intervals were analyzed (labeled “Entire Series” and “Short Series,” respectively, in Fig. 3).
The Lomb-Scargle Fourier transform method was applied using the SPECTRUM program, which allows analysis of unevenly spaced time series without interpolation (Schulz and Statteg-ger, 1997). We conducted univariate autospec-tral analysis (Welch method) to determine the dominant frequencies in the record. We also conducted harmonic analysis using Siegel’s test to discriminate periodic components from noise in the Zhada record. Cross-spectral analysis was used to determine the coherence between the Zhada record and the record of summer insola-tion for 65°N (Laskar et al., 2004).
Stable Isotopes
Stable isotopes of oxygen and carbon [expressed as δ18O and δ13C in units ‰, respec-tively, and referenced to Vienna Peedee belem-nite (VPDB) or Vienna standard mean ocean water (VSMOW)] are sensitive indicators of hydrologic conditions. The principal controls on surface water δ18O (δ18Osw) values in southern Tibet are increasing elevation (which decreases δ18Osw values) and evaporation (which increases δ18Osw values) (Dansgaard, 1954, 1964; Rozan-
1Supplemental Table 1. Word document containing data used in frequency analysis. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 1.
ski et al., 1993; Garzione et al., 2000b; Poage and Chamberlain, 2001; Rowley et al., 2001; Rowley and Garzione, 2007). Freshwater gas-tropods precipitate shells with oxygen isotopic ratios (δ18Occ [Shell carbonate oxygen isotope ratio]) in equilibrium with ambient water, depen-dent on the temperature-dependent fractionation factor (Fritz and Poplawski, 1974; Leng et al., 1999) between aragonite and water. The δ13C values of gastropod shells (δ13Ccc) are controlled by the δ13C value of dissolved inorganic carbon (δ13CDIC) in the ambient water (Lemeille et al., 1983; Bonadonna et al., 1999; Leng et al., 1999). The δ13CDIC value is controlled primarily by the residence time of water and secondarily by fac-tors including the local vegetation and substrate. The δ13CDIC value of surface water is increased by photosynthesis or equilibration with the atmosphere (Talbot, 1990; Li and Ku, 1997). Particularly in productive lakes, increased water residence time increases the δ13CDIC value. Thus, both δ13Ccc and δ18Occ values of gastropod shells are useful in reconstructing paleohydrologic and paleoenvironmental conditions (e.g., Abell and Williams, 1989; Purton and Brasier, 1997; Haile michael et al., 2002; Smith et al., 2004).
Fossil gastropod shell fragments and intact shells were collected from fl uvial, marshy, and lacustrine intervals from the lower ~650 m in 2 measured sections. Shells were powdered and homogenized prior to analysis. To check for preservation of biogenic aragonite, 12 repre-sentative gastropod samples from fl uvial, lacus-trine, and marshy intervals were powdered and analyzed using the University of Arizona’s D8 Advance Bruker X-ray powder diffractometer (Saylor et al., 2009).
We measured δ18Occ and δ13Ccc values using an automated carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252). Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70 °C. The isotope ratio measure-ment is calibrated based on repeated measure-ments of NBS-19 and NBS-18 and precision is ±0.1‰ for δ18O and ±0.06‰ for δ13C (1σ).
RESULTS
Sedimentology
We identify 14 lithofacies associations and fi ve depositional-environment associations based on lithology, texture, and sedimentary
structures (Supplemental Table 22). Unless otherwise indicated, all deposits are laterally continuous for hundreds of meters to several kilometers. Only abbreviated descriptions and interpretations are presented here (for details, see Saylor, 2008).
Depositional Cycles in the Zhada Formation
Deposits in the Zhada Formation occur in two types of cycles that mark periods of lake or wet-land expansion and contraction. The bulk of a typical type A cycle (Figs. 4A and 5A) consists of a 1–10-m-thick unit of fl uvial or alluvial fan sandstone or conglomerate (lithofacies associa-tion F1 or rarely A1–A4) with an erosional base, no grain-size trend, and a capping, upward- fi ning sandstone bed (lithofacies association F2 or occasionally S1). This is overlain by an organic-rich, fi ne-grained unit that contains con-voluted bedding (lithofacies association F3).
An idealized type B cycle (Figs. 4B, 5B, and 5C) is characterized by an upward-coarsening succession of, in ascending order, fossil-rich siltstone (lithofacies association L1), lami-nated or massive siltstone or sandy turbidites (lithofacies association P1–P2), rippled and cross-stratifi ed sandstone (lithofacies associa-tion L2), sandstone containing planar, trough, or climbing-ripple cross-stratifi cation (lithofa-cies association F2 or S1–S3), and conglom-erate beds (lithofacies associations A1–A4 or F1). The uppermost sandstone beds have both erosional and gradational basal surfaces. The basal surface of the capping conglomeratic unit is either erosional or marked by soft-sediment deformation. Organic-rich convoluted siltstone (lithofacies association F3) can occur at any point within the capping sandstone or conglom-erate succession. In all cases, the boundary between the fl uvial or alluvial fan association and littoral or profundal association is abrupt, while the transition from the profundal asso-ciation to the fl uvial or alluvial fan association is gradational, indicating a rapid transgression followed by gradual progradation. Parts of both type A and type B cycles may be missing from the idealized version depicted in Figure 4.
Correlations
Type A and type B cycles stack in predictable patterns within a larger sequence stratigraphic
2Supplemental Table 2. Word document containing the lithofacies association codes, descriptions, and interpretations. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 2.
Saylor et al.
78 Geosphere, April 2010
End leg 10 atN 31˚ 22.910’E 79˚ 45.075’4299 ± 8 m
End leg 9 GPS unavailable
Start leg 10 atN 31˚ 23.112’E 79˚ 44.981’4197 ± 6 m
n=10
n=28
n=19
End leg 8 atN 31˚ 24.449’E 79˚ 45.342’4057 ± 13 m
Start leg 9 atN 31˚ 24.158’E 79˚ 45.442’4057 ± 6 m
P
n = 17
n = 19
n = 14
End leg 7 atN 31˚ 25.280’E 79˚ 44.916’4001 ± 10 m
Start leg 8 atN 31˚ 24.584’E 79˚ 45.371’4001 ± 9 m
End leg 6 atN 31˚ 25.281’E 79˚ 44.986’3966 ± 7 m
Start leg 7 atN 31˚ 25.275’E 79˚ 44.989’3966 ± 6 m
n = 13
n = 16
Start leg 5N 31˚ 26.260’E 79˚ 45.143’3875 ± 7 m
End leg 5 atN 31˚ 26.121’E 79˚ 45.421’3929 ± 10 m
Start leg 6 atN 31˚ 25.507’E 79˚ 45.118’3933 ± 8 m
South Zhada (SZ) lithologic section Depo-codes
Record of summer insolationat 65°N (Laskar et al., 2004)
CS SS Cgm
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)
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Unidirectional ripples
Hummocky cross- stratification
Erosional surface
Plants/Plant fragments
Gastropods
Bivalves
Ostracods
Root Traces
Fish Skeletons/Fragments
Convoluted bedding/soft sediment deformation
Shell fragments
Mud cracks
Paleocurrent direction from trough cross beds
Paleocurrent direction from imbricated clasts
n = 17 Number of paleocurrent measurements
Paleocurrent direction from groundwater tubes
Climbing ripples
Terrestrial Mammal Fossils
Ent
ire
Ser
ies
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rt S
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s
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Sc
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Ml
Mh
Mh
Mh
Mh
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1 2 3 4 5Depo-codeLake/wetland expansion(Stronger
monsoon)
-60-40-20 0 20 40 60
2.5
3
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5
Variation in Insolation (W/m2)
Age
(M
a)
0 0.02 0.04 0.06
Eccentricity
Figure 3. The synthetic wave form constructed for spectral analysis. Depositional codes relate to lithofacies associations (5— alluvial fan and fl uvial asso-ciations; 4— supralittoral asso-ciations; 3—littoral associations; 2 or 1—profundal associations, based on the presence or absence of terrigenous clastic or biologic material). At ages younger than 3.3 Ma, the waveform saturates at values of 5 due to the infi ll-ing of the Zhada paleolake and the progradation of lake-margin depositional environments. Simi-larly, the inability to distinguish fl uctuations in water level dur-ing times of profundal or alluvial fan and/or fl uvial sedimentation results in clipping of the wave-form. The record of insolation variation (Laskar et al., 2004) is provided for comparison. GPS—global positioning system; C—claystone; S—siltstone; SS—sandstone; Cgm—conglomerate.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 79
hierarchy (Fig. 6). Because of the diffi culty of establishing a hierarchy of continental sequences based on sequence duration as deter-mined in marine sequences (e.g., Vail et al., 1991), we follow Catuneanu (2006) and estab-lish a unique hierarchy for the Zhada basin. The Zhada Formation does not have signifi cant intraformational unconformities that might represent extended periods of nondeposition or extensive subaerial exposure and erosion. However, it does have Waltherian unconformi-ties that represent rapid progradation of basin
margin facies and occasionally nondeposition of a lithofacies association. It is on the basis of these minor unconformities and associated shifts in depositional environments that we defi ne sequences of all orders.
At the fi nest scale, 56 type A and type B cycles are present in the Zhada Formation (Fig. 7). Four second-order sequences are evident above the cycles described above. Nomenclature used identifi es sequence order, systems tract or bounding surface, and strati-graphic position from lowest to highest (hence
2HST2 is the second-order highstand systems tract second from the base of the Zhada For-mation). Second-order lowstand systems tracts (2LST) are characterized by type A cycles arranged in a retrogradational (landward step-ping of depositional settings resulting in an increase in lake and/or wetland area) stacking pattern (Fig. 6). They are fl uvially dominated and become increasingly marshy upsection. Second-order transgressive (fl ooding) surfaces (2TS) are identifi ed by an abrupt transition to thick, profundal claystone (Fig. 8). Modern
Sandstone or conglomerate Massive or laminated siltstone or claystone Massive, laminated or rippled siltstone Papery laminated siltstone or claystone
III.Zhada Co(lake)
Alluvial fan
Paleo-Sutlej River
Marshy wetlands
submerged lake margin grasses
II.
Possible locations of Type A cycles
Type B cycle
I.
Fluvial or Alluvial fan Association
Supra-littoral AssociationFluvial or Alluvial fan Association
~ 10
m
A Type A cycle
Fluvial or alluvial fan deposition(Panel I.)
Wetland deposition (Panel II.)
Exposure or sediment bypass (Panel III.)
C Depositional Setting Panels
Profundal Association
Littoral Association
Supra-littoral Association
Fluvial or Alluvial fan Association
Fluvial or Alluvial fan AssociationLittoral Association
Low energy transgression drowns lake-margin semi-aquatic grasses (Panel I.)
Gradual progadation(Panel II. - III.)
{Sub-aerial exposure(Panel III.)
1 -
10 m
B Type B cycle
Profundal deposition(Panel II.)
D Lake level, water influx, and lake water δ18O
ProgradationProgradation Flooding Flooding
Greater lake area
Depo-setting Panels:
Netwater influx
Greater δ18O
values Time (t)
III IIIIII III
+
-
Figure 4. (A, B) Idealized forms of sequence types A and B. (C) Interpreted depositional environments. (D) Simplifi ed representation of the relationship between lake level, water, and sediment fl ux and lake δ18O values. The simplifi cations involve the assumption that end-member infl ux and effl ux δ18O values are invariant and that effl ux via evaporation is proportional to lake area. Vertical gray boxes indicate the time of retrogradation (sediment infl ux < water infl ux) associated with fl ooding. The black star denotes the tem-poral location of Kungyu Co within the systems tracts at the time of sampling (25 July 2006). The legend for sedimentary structures is found in Figure 10.
Saylor et al.
80 Geosphere, April 2010
Tibetan lakes are typically broad and shallow, and occur on low-relief plains. The lateral con-tinuity of depositional units implies that these conditions also existed during deposition of the Zhada Formation. When transgression occurred, it would have quickly fl ooded the depositional plain, resulting in rapid retrogra-dation. As a result, second-order transgressive systems tracts (2TST) are thin. They are char-acterized by type B cycles arranged in a ret-rogradational stacking pattern and are capped by widespread profundal lacustrine sedimen-tation. Second-order highstand systems tracts
(2HST) are characterized by type B cycles arranged in prograding (basinward stepping) or aggrading (no stepping pattern) stack-ing patterns. At the coarsest scale, the entire Zhada Formation can be seen as a fi rst-order sequence (approximately third-order sequence of Vail et al., 1977). Tract 1LST is below the fi rst major lacustrine transgression and is composed of 2LST1 and 2TST1. Tract 1TST occurs between the fi rst major lacustrine trans-gression and the most widespread profundal lacustrine sedimentation (maximum fl ooding surface) and is composed of 2HST1, 2LST2,
and 2TST2. Tract 1HST occurs between the most widespread maximum fl ooding surface and the top of the Zhada Formation and is composed of 2HST2, 2LST3–2HST3, and 2LST4–2HST4.
Type A and type B cycles can be corre-lated from stratigraphic sections spanning the entire thickness of the Zhada Formation (South Zhada and Guga sections) toward the basin margins. Sediment accumulation was great-est in the region of the South Zhada and Guga sections. However, the maximum thicknesses of fi ne-grained material were deposited to the
C Type B cycles
B
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Figure 5. (A) Type A cycles. Cliff is ~15 m high. (B) Photomosaic of typical progradational sequences in the lacus-trine portion of the Zhada Formation (Nl). Each cycle in B is ~ 10 m. However, the focus of photo B is to show the lateral continuity of the Zhada deposits. (C) Type B cycles. Lowermost cycle is ~9 m high. Lower slope-forming interval represents upward-coarsening profundal to littoral mudstones and siltstones (P1, L1, L2), which are capped by cliff-forming littoral or supra-littoral sandstones (L2 or S1).
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 81
northwest of there, in the region of the Namru Road West section. The implication is that, though relative subsidence was greatest in the region of the South Zhada and Guga sections, these were also close to the source of coarse-grained material (identifi ed in Saylor, 2008, as both the Kailash region to the north of the basin and also the mountain ranges immedi-ately surrounding the basin).
Frequency Analysis of Zhada Formation Cycles
The best time control based on magneto-stratigraphy in the Zhada basin is between chrons 2An (2.581 Ma) and 3n (5.23 Ma) (Lou-rens et al., 2004). There are 28 type B cycles within this interval, each with an average dura-tion of 95 k.y. Spectral analysis of both the
entire series and the 5.23–3.3 interval indicates statistically signifi cant peaks at 91.7 k.y. at the 95% confi dence level and at 22.4 k.y. at the 85% confi dence level (Fig. 9A). Harmonic anal-ysis of the entire series reveals peaks at 91.7 ± 2, 126 ± 4, 140 ± 4, 221 ± 12, 379 ± 40, 662 ± 287, and 1330 ± 2000 k.y. at the 99% confi dence level (Fig. 9B). However, in the analysis of the 5.23–3.3 Ma interval all of these peaks, except for
C S SS
C S S CgmS
Tethyan fold-and-thrust belt (basement)
1TS
1MFSS.E. Zhada S. Zhada
: Type A Cycles
: Type B Cycles
Figure 6. Portion of Figure 7 showing detailed parasequence scale correla-tions. See Figures 7 and 10 for leg-end. 1TA—fi rst-order transgressive (fl ooding) surface; 1MFS—fi rst-order maximum fl ooding surface.
Saylor et al.
82 Geosphere, April 2010
CS
SC
gm
SC
SS
Cgm
S
CS
SC
gm
S
Sh
Sf
Sf
St
St
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Ml
Ml
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Ml
CS
SC
gm
S
CS
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S
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n (d
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7 (c
ontin
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). B
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and
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ence
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atig
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ons.
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orth
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th tr
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ct.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 83
CS
SC
gm
S
CS
SC
gm
S
CS
SC
gm
S
CS
SC
gm
S
CS
SC
gm
S
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t)
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ance
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d
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20km
10km
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10km
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10km
8km
8km
3 N
.W. Z
hada
2 N
.W. Z
hada 1 N
.W. Z
hada
Qus
um
50 m
vert
ical
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onal
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ontin
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) Sou
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e F
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S—tr
ansg
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ce; M
FS—
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—se
quen
ce b
ound
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ghst
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ems
trac
t; L
ST—
low
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oc.—
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—tr
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ems
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ssoc
.—as
soci
atio
n.
Saylor et al.
84 Geosphere, April 2010
the 379 and 91.7 k.y. peaks, are suppressed (Fig. 9C). This indicates that the suppressed peaks are likely the result of red noise due to wave-form saturation at ages younger than 3.3 Ma. -Both the entire series and the shorter interval pass Siegel’s test, indicating that the record is not the result of white noise. A random time series of similar length showed no statistically signifi cant peaks and did not pass Siegel’s test. Coherence analysis of the shorter interval also reveals peaks at 80 ± 13, 26 ± 1, and 18.9 ± 0.6 k.y. (Fig. 9D).
Stable Isotopes
The X-ray diffraction analyses from 11 of 12 samples yielded only aragonite peaks (Saylor et al., 2009); the 12th sample was too small to yield results. The δ18Occ values of samples that we analyzed using X-ray diffraction ranged from −20.3‰ to +0.2‰ (VPDB).
Figure 8. Second-order transgressive surface showing the abrupt transition from lithofacies association S1 and F2 sandstone to lithofa-cies P1 profundal claystone. Homo sapiens (circled in red) for scale.
DFigure 9. (A) Power spectrum of the entire interval (5.23–2.581 Ma). The spectrum has peaks at ~100 k.y. at the 95% confi dence level (CL) and ~23 k.y. at the 85% confi dence level. Y axis is log (base 10) of relative power. (B) Harmonic analysis of the entire interval reveals dominant peaks at 379 and 91.7 k.y., but also has signifi cant red noise. (C) Harmonic analysis of the interval 5.23–3.3 Ma reveals the same dominant peaks, but red noise peaks are signifi cantly suppressed. (D) Coherence analysis reveals a peak at 91 k.y. Vertical error bars indicate the 95% confi dence interval.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 85
Clearly identifi able trends in multiple cycles were found only in the densely sampled South Zhada section, particularly in the 250–470 m interval of the South Zhada section where we focus our discussion. (For analysis of the entire data set, see Saylor et al., 2009.) The δ13Ccc val-ues of gastropods in this interval range from –3.3‰ to +2.1‰ (VPDB), and δ18Occ values are from −13.7‰ to +0.7‰ (VPDB; Table 1).
There are 17 type B cycles within the 250–470 m interval of the South Zhada section (Fig. 10). Of those, eight had suffi cient sampling den-sities that trends in δ18Occ values should be evi-dent. Five cycles show a clear trend of increas-ing δ18Occ values with stratigraphic height above the cycle boundary (Fig. 10). One additional cycle shows a similar, but muted, trend (Fig. 10). The fi nal two cycles do not show any trend in δ18Occ values (Fig. 10).
INTERPRETATION OF ZHADA FORMATION CYCLES
Zhada Formation type A and type B cycles are best interpreted as parasequences (a conformable succession of beds separated by fl ooding sur-
faces; Van Wagoner et al., 1988b). Parasequences are typically thin (<20 m) and correspondingly short-lived (~100 k.y.). We conclude that facies are controlled primarily by lake or wetland expansion and contraction, which are related by the interplay of sedimentation and base-level change at the shoreline. This is most evident in type B parasequences where fl ooding surfaces are easily identifi able as the sharp basal contact of the fi ne-grained, fossil-rich, often papery interval capping a coarser-grained unit (Fig. 4B). In type A parasequences fl ooding is probably recorded by the transition from the fl uvial association to marshy deposits of the supra-littoral association rather than by an abrupt surface as in type B parasequences (Fig. 4A). However, type A para-sequences have clearly identifi able erosive sur-faces that can be correlated to subaerial exposure surfaces in type B parasequences (Figs. 4, 5, and 6). Thus, the maximum regressive surface in both type A and type B parasequences is defi ned as the erosional surface at the base of the coarsest-grained interval, if an erosional surface is present, or at the base of the lowest sandy interval show-ing signs of unidirectional traction transport if no erosional surface is present.
Type A parasequences occur at the base of the Zhada Formation sequences (Fig. 6). Marshy deposits become more prominent com-ponents of type A parasequences higher in the sequences, consistent with the general retrogra-dational stacking pattern. The upward-fi ning textural trend, retrogradational stacking pattern, and location at the base of the Zhada Formation sequences suggest that type A parasequences represent onset of lacustrine transgression. The associated rise in the water table resulted in increased marshy conditions, although the system was still dominated by fl uvial processes (e.g., Bohacs et al., 2000).
Type B parasequences occur in the middle to upper Zhada Formation (Fig. 6) and coarsen upward from a profundal lacustrine lithofacies association to a supralittoral or fl uvial lithofa-cies association. Thus, they represent progra-dational parasequences in a lacustrine setting. The persistence of these cycles to the top of the Zhada Formation indicates that lacustrine conditions prevailed until the onset of incision by the modern Sutlej River, despite prograda-tion causing replacement of the fi ne-grained littoral or supra-littoral deposits by basin- margin alluvial fans.
Zhada Formation cycles obey Walther’s Law. Within individual cycles, facies that are super-posed occurred side by side spatially (e.g., Mid-dleton, 1973; Posamentier and Allen, 1999). This is consistent with sequence stratigraphy theory (Van Wagoner et al., 1988b) but contrasts with reports of non-Waltherian cycles from the Green River Formation and underfi lled lacus-trine basins in the Qaidam basin and Death Val-ley (Yang et al., 1995; Lowenstein et al., 1998; Pietras and Carroll, 2006).
DISCUSSION
Sequence Stratigraphic and Lithostratigraphic Correlations
The overfi lled, balanced-fi lled, and under-fi lled intervals of the Zhada basin were delin-eated using defi nitions modifi ed from Bohacs et al. (2000). In contrast to the evaporative facies association presented by Bohacs et al. (2000) as typical of underfi lled lake basins, evaporites are present, though not dominant within the Zhada sections. It may be argued that no sections were measured in the basin center and so the possi-bility exists that that is the locus of evaporite deposition. However, that is unlikely given the lateral facies continuity in the Zhada Formation and the number of measured sections close to the basin center. A more plausible explanation is that discharge into the basin by the paleo–Sutlej River was consistent and large enough that the
Saylor et al.
86 Geosphere, April 2010
-20-18-16-14 12- -10 -8 -6 -4 -2 0
Str
atig
raph
ic h
eigh
t (m
)
δ18O (VPDB%)
End leg 8 atN 31˚ 24.449’E 79˚ 45.342’4057 ± 13 m
Start leg 9 atN 31˚ 24.158’E 79˚ 45.442’4057 ± 6 m
n = 14
End leg 7 atN 31˚ 25.280’E 79˚ 44.916’4001 ± 10 m
Start leg 8 atN 31˚ 24.584’E 79˚ 45.371’4001 ± 9 m
End leg 6 atN 31˚ 25.281’E 79˚ 44.986’3966 ± 7 m
Start leg 7 atN 31˚ 25.275’E 79˚ 44.989’3966 ± 6 m
n = 13
n = 16
End leg 5 atN 31˚ 26.121’E 79˚ 45.421’3929 ± 10 m
Start leg 6 atN 31˚ 25.507’E 79˚ 45.118’3933 ± 8 m
Sh
Sh/St
St
St
St
St
St
Mh
St/Sh
Mh
Mh
Mh/Sh
Mh
Ml
Mh
Mh
Mh
Ml
Mh
St
St
St
St
St
St
St
St/GctSt
St
Sr
St
Sr
Sr
Mr/Sr
Mh
Sf
Sh
St
Mh
Ml
MhMl
MhMl
MhMl
Mh
Mh
MrSr
MhMl
Mh
StMh
Mh
Mh
Sh
Ml
Ml/St
Ml
Ml
Mh/Sr
Sr/Sc
Sr/Sc
Sr
Sr
Sr
Ml
380
360
300
260
240
280
320
340
C S S CgmS
P
n = 17
n = 19
Sh
St
St
Sr
Sr
StSt
St
St
St
St
St
St
GmmSc
Sc
St
Sf
Sh
St
Ml
Mh
Mh
Mh
Mh
Mh/Sr
Sr
Sr
Mh
Mh
Mh
MlMh
Mh
Mh
Mr/Mc
ShMh
MrGct
Mr
Hcs
460
440
420
400
480
Legend
Lithofacies
Oscillatory current ripples
Unidirectional ripples
Hummocky cross- stratification
Erosional surface
Plants/Plant fragments
Gastropods
Bivalves
Ostracods
Root traces
Fish skeletons/fragments
Convoluted bedding/soft sediment deformation
Shell fragments
Mud cracks
Paleocurrent direction from trough cross bedsPaleocurrent direction from imbricated clasts
n = 17 Number of paleocurrent measurements
Paleocurrent direction from ground water tubes
Climbing ripples
Terrestrial mammal fossils
Rip-up clasts
Gcmi
Gm
Gt
St
Sr
Sh/Sm
Sf
Sc
Mr
Ml
Mh
Mm
Mc
Figure 10. South Zhada lithologic section and associated δ18O values of aquatic gas-tropods. Horizontal black lines represent parasequence boundaries. Thick verti-cal green boxes indicate the sequences that were used to construct Figure 12. Within all fi ve sequences where a trend is evident, δ18Occ values increase from the fl ooding surface to the maximum regression surface. Vertical orange boxes indicate sequences that show a possible, though not clear, trend. Vertical red boxes indicate sequences with suffi cient sampling density that trends in δ18Occ val-ues may be expected, but where no trends are observed. VPDB—Vienna Peedee belemnite; C—claystone; S—siltstone; SS— sandstone; Cgm—conglomerate.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 87
lake rarely desiccated though surface outfl ow was minimal (e.g., Lake Naivasha; Barton et al., 1987; Duhnforth et al., 2006).
The overfi lled interval was determined based on the prevalence of fl uvial input, indistinctly expressed parasequences (Bohacs et al., 2000), and the dominance of sedimentary structures indicating traction transport. The overfi lled interval extends from the base of the section to 1TS (which is the same surface as 2TS1; Fig. 7). The δ18Occ values from this interval are extremely negative due to the low water- residence times associated with river through-fl ow (Saylor et al., 2009).
The balanced fi ll interval is identifi ed by a dominantly retrogradational parasequence stacking pattern. The balanced fi ll interval is characterized primarily by the rising water table inferred from the increased prevalence of marshy intervals. Though the basin was inter-mittently open, fl uvial infl ux was greater than effl ux via outfl ow and evaporation and so the basin was being slowly drowned. The balanced fi ll interval extends from 1TS to 2MFS1 (Fig. 7). The trend toward more positive δ18Occ values in this interval and the inferred increase in water residence times (Saylor et al., 2009) are consis-tent with this interpretation.
The underfi lled interval is well represented in the Zhada Formation and was identifi ed based on the occurrence of well-expressed fl ood-ing surfaces that separate distinct lithologies. Parasequences are well developed and record a combination of progradational and aggrada-tional stacking patterns. Depositional geome-tries (fl ooding surfaces) are generally parallel or subparallel and well-expressed parasequences converge and become indistinct toward the basin center. Within parasequences, trans-gressive deposits are thin (<0.5 m) or absent, whereas progradational deposits are thick, well developed, and dominated by traction transport (oscillatory current ripples, climbing ripples). The underfi lled interval extends from 2MFS1 to the paleodepositional surface.
Type B parasequences occur primarily in the underfi lled portion of the Zhada basin. How-ever, they differ from previous descriptions of underfi lled basin lithofacies (e.g., Carroll, 1998; Bohacs et al., 2000). The primary dif-ference is that coarse-grained facies were pre-sented as the result of transgression by Bohacs et al. (2000) and Carroll (1998), whereas in the Zhada basin they typically constitute the regressive portion of the parasequence. There are several reasons for interpreting coarse-grained facies as the regressive part of the cycle in the Zhada basin. Unlike the cycles presented by Bohacs et al. (2000) and Carroll (1998), fi ne-grained, subaerial exposure surfaces do
not directly underlie the coarse-grained facies. Rather, the profundal lacustrine facies coars-ens upward gradually and shows evidence of traction transport, including oscillatory current ripples, throughout regression. The coarse-grained facies exhibit evidence of subaerial exposure including preferential weathering and cementation, and root traces. In addition, the coarse-grained facies are often interbed-ded with organic-rich siltstone and sandstone facies (lithofacies association F3) indicative of marshy wetlands, such as might occur on lake margins or between fl uvial channels. We therefore interpret the coarse-grained facies as the maximum progradation of lake-margin depositional environments (Figs. 4C, 4D, panel I). One possible explanation for the difference between type B cycles and those in the basins studied by Bohacs et al. (2000) and Carroll (1998) is that fl uctuations in infl ux were not as great in the Zhada basin, and that the Zhada
basin rarely became desiccated. If water, and thus sediment, infl ux were relatively stable, regression would be marked by progradation and, during maximum regression, which marks the time when the lake has the smallest volume and is the most restricted, the relative infl uence of fl uvial input would be greatest.
The evolution of the Zhada basin followed a typical pattern from a fl uvial system to an underfi lled lacustrine basin (Fig. 11) (Bohacs et al., 2000). However, the top of the Zhada For-mation is dominated by coarse-grained, basin-margin equivalents of type B sequences. There is no change in large-scale sedimentary environ-ment indicated prior to an abrupt truncation of the Zhada Formation by a paleodepositional plain. By implication, there was no return to a balanced fi ll or overfi lled basin type. The return to fl uvial conditions often observed was discon-tinuous in that it bypassed the balanced fi ll and overfi lled intervals (Fig. 11).
Flu
vial
Eolian
Overfilled
Balanced fill
Underfilled
Thick Source Intervals
Thin Source Intervals A B
C
Accommodation (height of sill above base level)
Sed
imen
t and
wat
er s
uppl
y
Accom
modation > supply
Accommodation < supply Accommodation supply
=~
Figure 11. The trajectory of Zhada basin evolution in accommodation and sediment-supply and water-supply space. Also shown are fi elds occupied by over-fi lled, balanced fi ll, and underfi lled basins. The Zhada basin followed a typical evolutionary pattern from fl uvial to underfi lled basin due to an increase in accom-modation (solid black arrow A) until a new sill was breached (B). At this point the basin underwent a discontinuous return to fl uvial conditions, bypassing the usual progression back through the balanced fi ll and overfi lled fi elds (dashed black arrow C). Modifi ed from Bohacs et al. (2000).
Saylor et al.
88 Geosphere, April 2010
Frequency Analysis
The two independent time-series analyses described above indicate that ~100 k.y. cycles are present in the Zhada Formation. In addition to a peak at 91.7 k.y., univariate spectral analy-sis reveals a peak at 22 k.y. These are within 1/2 bandwidth (6 dB bandwidth = 2.4) of the eccentricity and precession frequencies. Har-monic analysis does not reveal the 22 k.y. peak indicated by univariate analysis, but does show peaks at 91.7 and 379 k.y., both of which are consistent with the eccentricity cycle (Figs. 9B, 9C). Coherence analysis shows coherence with both eccentricity and insolation records (Laskar et al., 2004) only at the eccentricity frequency (Fig. 9D). The fact that both frequency analysis and an average cycle duration shows 100 k.y. cyclicity indicates that this signal is robust.
Sequences and parasequences in the Zhada Formation are either tectonic or climatic in origin. The correlation between the fi rst-order transgressive surface (Fig. 7, 1TS) and major tectonic reorganization in the Zhada region (Saylor, 2008) points to a tectonic origin for the fi rst-order sequence. Likewise, the correlation between the fi rst second-order transgressive surface (Fig. 7, 2TS1) and maximum fl ooding surface (Fig. 7, 2MFS1) with the major tectonic reorganization and an increase in the exhuma-tion rate on of the Leo Pargil Range, respectively, (Thiede et al., 2006; Saylor, 2008) also points to a tectonic origin for second-order sequences. The number and consistent and short duration of parasequences rule out a tectonic origin. Parasequences are consistent in duration with
insolation-driven climate changes (fourth-order sequences of Vail et al., 1977) due to changes in the orbital characteristics of the Earth (i.e., Milankovitch cycles). If parasequences are rep-resentative of Milankovitch cycles, the driving process behind high-frequency environmental cyclicity in the Zhada basin was not tecton-ics. Rather, lacustrine expansion and contrac-tion was caused by a change in the long-term precipitation to evaporation ratio. Long-term changes in the precipitation/evaporation ratio have been linked to strengthening or weakening of the monsoon due to increases or decreases, respectively, in insolation (Kutzbach, 1981; Prell and Kutzbach, 1992; Gupta et al., 2001; Shi et al., 2001; Ruddiman, 2006; Thompson et al., 2006). Shi et al. (2001) suggested a causal link between monsoon strength and Tibetan lake expansion and, in the absence of a change in winter rainfall in Tibet, we link Zhada paleo-lake size to insolation-driven monsoon inten-sity. It is not surprising that climatically driven parasequences are most distinctly expressed in the underfi lled interval of the Zhada Formation, because during this interval the lake would be most susceptible to changes in hydrology (Kelly, 1993; Bohacs et al., 2000).
Isotopes in Zhada Formation Cycles
Lakes respond to changes in hydrology on much shorter time scales than do oceans because they have much smaller water and sediment vol-umes (see Kelts, 1988; Sladen, 1994; Bohacs et al., 2000). In addition, in lacustrine settings the relative proportions of water infl ux and effl ux
(usually climatically driven) and movement on faults (tectonically driven) are both primary controllers of systems tracts. Sediment supply is linked to water infl ux. This creates the para-doxical situation where the infl ux of water and lake volume can be high, and yet lake volume can be decreasing (net evaporation > net infl ux; Figs. 4C, 4D).
Water and sediment infl ux are thus decoupled from base-level changes but are primary con-trollers of shoreline trajectory, and therefore parasequence evolution. Lithofacies distribution and thus lithologic stacking patterns appear to be controlled primarily by the location of the shoreline and so are also decoupled from lake volume. This means that parasequence fl ood-ing surfaces correspond to lake expansion due to a drop in the evaporation/precipitation ratio. Thus, the lowest δ18Occ values of aquatic gas-tropods and, implicitly, of the lake water, are found at the fl ooding surface, even though the coarsest material is associated with maximum regression (Fig. 12). Particularly when the basin was underfi lled, the highest isotopic values occur at the time of maximum regression (Figs. 4D and 10). This apparent discrepancy can be accounted for by understanding that though water and sediment infl ux were both relatively high and stable, climatically driven evapora-tion/precipitation controlled lake level and thus the δ18Osw value of lake water. When effl ux was greater than infl ux, the δ18Osw value increased, the lake shrank, and the coarse-grained material was carried further into the basin (Fig. 4C, panel I). Conversely, when infl ux was greater than effl ux, the δ18Osw value decreased, the lake grew,
0
0.25
0.5
0.75
1
-4 -3 -2 -1 0 1 2 3
Nor
mal
ized
hei
ght a
bove
flo
odin
g S
fc
Parasequence 1
Parasequence 2
Parasequence 3
Parasequence 4
Parasequence 5
-16 -14 -12 -10 -8 -6 -4 -2 0 2
Flo
odin
g R
egre
ssio
n
δ13Ccc(‰ VPDB) δ13Ccc(‰ VPDB)
Figure 12. δ18O and δ13C values (Vienna Peedee belemnite, VPDB) of aquatic gastropods from fi ve sequences (indicated in Fig. 10) are plot-ted against their normalized height above the fl ooding surface (Sfc). The lowest values occur just above the fl ooding surface and represent lake expansion associated with a decrease in the evaporation/precipitation ratio. However, continued evaporative enrichment and isotopic evolution means that δ18Occ and δ13Ccc values increase through most of the regressive sequence.
Sequence stratigraphy and climate cycles in southwestern Tibet
Geosphere, April 2010 89
and the coarse-grained material was trapped at the basin margins (Fig. 4C, panel II).
The foregoing discussion indicates that the primary control of δ13Csw and δ18Osw values was volume-weighted average water residence time. Just prior to fl ooding, when lake vol-ume was small, the average water residence time, and hence δ13Csw and δ18Osw values, was signifi cantly altered by addition of a small vol-ume of water. However, after signifi cant fl ood-ing, the lake was suffi ciently large and the water suffi ciently evolved that the continued input of water during fl ooding had only a minor effect on average water residence time. Though the dis-cussion here refers primarily to individual para-sequences, the effect may span several parase-quences and point to climatic control at multiple frequencies (Fig. 10).
The correlation between low δ18Osw val-ues and fl ooding described here is confi rmed in the modern analog of Kungyu Co. Water samples collected on 25 July, 2006, from the lake and from the sole river fl owing into the lake had δ18O values of −14.8‰ and −15.6‰ (VSMOW), respectively. Stranded shorelines with aquatic grasses and evaporites on the lake margins showed that the lake was recently at higher levels. The samples were collected at the start of the monsoon season and the interpreta-tion is that the lake level had fallen to extremely low levels and was now in the process of refi ll-ing (Fig. 4D; black star denotes the interpreted location of Kungyu Co within the fi lling and/or emptying cycle at the time of sampling).
Basin History
Combining the observations made above with previous studies (Saylor, 2008; Saylor et al., 2009) points to the following basin history. Through arc-parallel extension, a sill was cre-ated that caused ponding of the river, leading to deposition of the lowest strata of the Zhada For-mation. The accumulating sediment onlapped the preexisting Tethyan sequence topogra-phy, forming the observed buttress or angular unconformities. The ancestral Sutlej River continued to fl ow from its source, increasing the sediment pile. The exhumation rate of the Leo Pargil–Qusum Range to the northwest of the Zhada basin between 10 and 5.6 Ma was the same as the sediment accumulation rate in the Zhada basin (Thiede et al., 2006; Saylor, 2008), indicating that the uplifting range may have acted as a sill. After 5.6 Ma both the exhu-mation rate and the sediment accumulation rate increased, the basin became closed, and lacustrine sedimentation commenced. These conditions continued, despite progradation of basin-margin alluvial fans, until a new sill was
eventually breached after 1 Ma. At this point, the system abruptly returned to fl uvial condi-tions and began incising through the Zhada Formation. The sudden return to fl uvial condi-tions via the integration of the modern Sutlej River system truncated the typical basin evolu-tion pattern described by Bohacs et al. (2000).
Global Climate Change and Its Impact on the Southern Tibetan Plateau
Numerous authors have reported 100 and 400 k.y. cycles in the Miocene (Van Wagoner et al., 1988a; Kashiwaya et al., 2001; Zachos et al., 2001; Di Celma and Cantalamessa, 2007; Holbourn et al., 2007), although none from high elevations such as the Tibetan Plateau. The Zhada basin therefore presents an excel-lent opportunity to study high-frequency cli-matically driven environmental change at high elevations in the Miocene–Pleistocene. Expan-sion and contraction of lakes and wetlands have been linked to variability in the strength of the Indian monsoon (Shi et al., 2001). The Quater-nary monsoon is thought to be modulated by orbital cyclicity (Clemens et al., 1991; Prell and Kutzbach, 1992; Jian et al., 2001; Wang et al., 2005; Nie et al., 2008; Y. Wang et al., 2008b), though there is disagreement about which fre-quencies are dominant (Clemens and Prell, 2003; Nakagawa et al., 2008). Data from this study support previous work indicating that the monsoon has long varied at eccentricity fre-quencies (Dupont-Nivet et al., 2007).
We turn next to another challenge presented by the Zhada basin, i.e., the explanation of the fl oral and faunal changes observed within the Zhada Formation and between the Late Mio-cene and the present. The Zhada basin con-tained a host of plants that are typically thought of as native to warm, humid, and, as inferred by some, low-elevation climates (Li and Zhou, 2001a, 2001b; Zhu et al., 2004, 2007). In addi-tion, a broad cross section of mammal mega-fauna lived in the Zhada basin area, including Hipparion zandaense, Nyctereutes, Palaeotra-gus microdon, and rhinoceri that have variously been identifi ed as Hyracodon or Dicerorhinus (Liu, 1981b; Zhang et al., 1981; X. Wang, 2006, personal commun.; E. Lindsay, 2006, personal commun.; Li and Li, 1990; Meng et al., 2004). This is in striking contrast to the basin today, in which the only large mammalian fauna are the kiang (Tibetan wild asses) and extremely rare chiru (small, long-horned antelope).
The recognition of Milankovitch cycles in the Zhada Formation indicates that insolation-driven global or regional climate change drove environmental changes in basins at high eleva-tions on the southern Tibetan Plateau. Thus,
we can reasonably expect that fl oral and faunal communities on the Tibetan Plateau would also have responded to global climate change. The shift from C3-dominated forests to mixed C3 and C4 or C4-dominated grasslands observed in the Zhada basin (Zhang et al., 1981; Zhu et al., 2006, 2007; Yu et al., 2007; Saylor et al., 2009) was not the result of basin uplift, because an identical change is observed in low-elevation deposits in nearby northern India. Further, anal-ysis of oxygen isotopes from aquatic gastro-pod shells from the Zhada Formation indicates a probable decrease in elevation of the basin since the Late Miocene (Saylor et al., 2009). A more likely scenario is that a regional or global climatic change affected both low- and high-elevation environments and favored a shift from forest to grassland.
A possible scenario is that the vegetation shift began at high elevations due to a global or regional climate change. Suggested factors include the onset of rapidly decreasing global temperatures in the latest Miocene–Pliocene (Zachos et al., 2001) or increased monsoon intensity (Kroon et al., 1991) and associated increased aridity and seasonality of precipita-tion (Guo et al., 2002; Garzione et al., 2003; Molnar, 2005). Increased warm-season precipi-tation and increased aridity favor C4 grasses (An et al., 2005). As is thought to be the case in the foreland, the fl oral shift was accompanied by faunal change at high elevations.
The possibility remains that these climate changes were driven by expansion of the region of high elevations, particularly on the northern and eastern margins of the Tibetan Plateau (e.g., An et al., 2001). However, any such models must take into consideration long-lived high elevations in the southern and central Tibetan Plateau (Garzione et al., 2000a; Rowley et al., 2001; Currie et al., 2005; Cyr et al., 2005; Row-ley and Currie, 2006; DeCelles et al., 2007; Dupont-Nivet et al., 2008; Saylor et al., 2009).
CONCLUSIONS
1. Lithologic cycles (types A and B) in the Zhada basin are Waltherian parasequences.
2. Sedimentology and sequence stratigraphic analysis indicate that the Zhada basin evolved from a fl uvial system to an overfi lled basin. The overfi lled basin was marked by a broad deposi-tional plain dominated by wetlands bordering a large braided river. From there, the basin evolved sequentially to a balanced fi ll basin and an under-fi lled basin. The fi nal stage was marked by open lacustrine conditions, which give way to pro-grading basin-margin alluvial fans. The typical regression through the basin-type sequence was bypassed by an abrupt return to fl uvial conditions.
Saylor et al.
90 Geosphere, April 2010
This agrees with overtopping of a basin sill and integration of the modern Sutlej drainage net-work (Brookfi eld, 1998; Saylor, 2008).
3. Two orders of sequences are recognized in the Zhada basin in addition to the parasequences mentioned herein. The fi rst-order sequence is the result of tectonically created accommoda-tion and infi lling; the second-order sequences are of ambiguous origin but may be linked to continued fault movement.
4. Where best expressed and dated, parase-quences have an average duration of ~92 k.y. and are likely the result of climatic changes associated with Milankovitch cycles. This is the fi rst time that 100 k.y. cycles have been reported for Late Miocene–Pliocene deposits on the Tibetan Plateau and presents an unparalleled opportunity to study high-frequency climate change at high elevations.
5. Within parasequences, the lowest δ18Occ values and, by implication, the lowest δ18O val-ues of Zhada paleolake water, are associated with fl ooding. From the fl ooding surface through maximum regression, δ18Occ values increase. This trend is the result of low evaporation/ precipitation ratios during fl ooding and the asso-ciated increase in lake volume and decrease in volume weighted average water residence time.
6. The data presented in this paper are con-sistent with a tectonic origin of the Zhada basin. Possible tectonic originating causes include crustal thinning or tectonic damming due to arc-parallel extension.
7. It is likely that regional or global climate change, rather than basin uplift, was the cause of the observed fl oral and faunal turnover in the Zhada basin. The turnover, marked by decreased arboreal pollen in favor of shrub and grass pol-len and a decline in the megafaunal populations, is similar in age and character to that observed in other basins on and surrounding the Tibetan Plateau. In the Himalayan foreland and else-where, the turnover is attributed to regional or global climate change. Though the introduction of C4 vegetation had previously been docu-mented on the southern Tibetan Plateau, the large-scale change from forest to grassland and the accompanying change in fauna observed in the low-elevation Siwalik Group had not been extrapolated to high elevations.
ACKNOWLEDGMENTS
We thank David Dettman and Majie Fan for assistance with stable isotope analyses, and our fi eld assistants, Cai Fulong, Jeannette Saylor, and Scott McBride. Reviews by an anonymous reviewer and J. Pelletier helped to signifi cantly strengthen this manuscript. Additional support was provided by the National Science Foundation Tectonics Program, ExxonMobil, Chevron-Texaco, and the Galileo Circle of the University of Arizona.
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